1 Field of the Invention
This invention relates to the forming of large objects, such as refrigerator cabinet liners, from crystalline synthetic resin, such as polypropylene.
2 Description of the Background Art
It has been long conventional to form large objects, such as refrigerator liners and the like, from formable synthetic resins, such as ABS plastic. The need for drawing the sheet material deeply to form the desired refrigerator liner configuration has precluded the use heretofore of crystalline synthetic resins for such large objects. It has long been known to form small objects from crystalline synthetic resins, such as polypropylene, but it has heretofore not been possible to form large objects from such materials while retaining the necessary control over shape and wall thickness as is required for refrigerator liners and the like.
It has been found that crystalline synthetic resins have limited hot strength at thermoforming temperatures, at which the resin is no longer crystalline. Thus, attempts to utilize the processes conventionally used in forming refrigerator liners from ABS sheet stock have been found to be totally impractical for use in forming large deep drawn parts from polypropylene sheet stock. The polypropylene was torn and pulled apart during the forming process. For a number of reasons, polypropylene is a preferred material for such refrigerator liners and the like, and it has long been a desideratum in the manufacture of refrigeration apparatus to utilize polypropylene if it could be formed in an economical and troublefree manner.
Recently, a process commonly referred to as solid phase forming has been developed as a forming technology wherein crystalline resins are carefully heated to a temperature adjacent but below the crystalline melting point of the thermoplastic resin, or polymer. In the solid phase forming methods developed to date, stretch forming has been suggested as a selected form of the solid phase forming method.
Because the resin is heated to a temperature below the crystalline melting point, this process is conventionally referred to as "solid phase forming". In carrying out the different proposed solid phase forming methods, some of the techniques employed have been generally similar to those employed in metal forming operations.
One of the problems of the conventional hot forming of thermoplastic resins has been the dissipation of the heat in setting the formed liner. A substantial advantage is obtained in the use of the solid phase forming process in that a smaller amount of heat need be removed from the formed object in completing the forming operation. This, in turn, permits increased production rates and, thus, lower cost in the manufacturing operations. As the manufacture of refrigeration apparatuses and the like is highly competitive, such reduced costs are a substantial consideration.
Because the polypropylene sheet is below the crystalline melting temperature, substantial force is required to form the sheet from the flat configuration into the deep drawn configuration of the conventional refrigerator liner and the like. In utilizing the lower temperature solid phase forming process, substantially different parameters and techniques have had to be ascertained and developed against a background in which it appeared that such solid phase forming processes were not adapted for use with large objects, but were capable of satisfactory use only with relatively small objects, such as food containers and the like.
Solid phase forming of the sheet stock may be effected by different specific techniques, such as stretch forming, pressure forming, stamping, drawing, and forging, as recognized techniques in the metal forming field. No one heretofore has been able to coordinate the different specific techniques in such a manner as to provide satisfactorily formed large objects having critical dimensions from the solid phase forming technique.
One prior method and apparatus for use in forming polypropylene is disclosed in U.S. Letters patent No. 3,546,746 of Herbert G. Johnson. As disclosed therein, the method and apparatus are adapted to effect solid phase working of plastics of the kind which can be strengthened by being worked in the solid phase. The invention of the Johnson patent is directed toward providing strengthening of the material by biaxial molecular orientation, as may be most suitable for the article produced.
Briefly, as disclosed in the Johnson patent, the method and apparatus are intended for use in making flanged hollow articles from a flat sheet. The outer edge of the blank is gripped about its periphery against a seat in an axial peripherally confined recess by a suitable clamp. An inner portion of the blank is moved axially as inventory material by a plunger moving through an annular orifice. The resulting product comprises a hollow article having a reduced thickness top flange, and a sidewall which is solid phase formed by controlled stretching thereof.
The clearance between the plunger and the sidewall of the female die defines the maximum thickness of the sidewall of the article. Means are provided for supplying to or removing fluid from one or both sides of the blank, as part of a system for controlling the flow of the material and also removing the formed article.
The Johnson patent further discloses the maintenance of a bottom portion of the molded product to form a closed bottom container with a top-strengthening rim. The rim is compressed by the clamping operation so as to cause it to be reduced in thickness from the original stock and to deliver some of the material therefrom radially inwardly to provide a thickening of the stretched portion of the product adjacent the rim. The compression of the rim causing the peripheral flow provides a biaxial orientation thereat for improved resistance to shock forces. Johnson teaches the heating of the prevailingly isotactic polypropylene to a temperature in the range from about 150.degree. F. to just below the crystalline melting point of 335 .degree. F., with the preferred range being between about 300.degree. F. and about 330.degree. F. The patentee teaches the withdrawing of the plunger while permitting the article to remain in the female mold. Pressurized fluid is delivered through the plunger to aid in the separation and, subsequently, pressurized fluid is delivered through the female mold to assist removal of the formed article therefrom.
Prior to forming the sheet, the patentee teaches the heating of the sheet to a substantially uniform temperature not varying by more than 2.degree. F. from the desired value across the surface of the blank. Working pressures are disclosed as being from about 100 to 500 psi, or more, and a holding time of about 1 to 15 seconds is disclosed as being sufficient to set the material and avoid springback.
Typical solid phase formed plastic articles produced by the techniques of the prior art have included containers, boxes, cases, pails, shovels, pipe flanges, valves and fittings, knobs, handles, washers, gears, pump motors, cams, pulleys, wheels and bearings, rivets, nuts, bolts, and the like. Polypropylene has been solid phase formed in forming meat, salad, and dairy tubs, portion packs, drinking cups and juice containers, and food service trays. The technique has been utilized in forming oil cans and, in one prior use, a five-gallon shipping container having a diameter of 12" and a height of 13" was so formed from the polypropylene sheet stock.